If you ask around you will get many different opinions on whether your frame will need to be pinched for a 1930 - 1931 body. Although the one thing you must consider is a '30 - '31 Ford cowl is 1 1/2" narrower at the bottom than a 1932 Ford.  Many people choose to build their car the way it was done in the "old days."  One technique would be to sit the body on top of the frame with a wood spacer from the firewall to the wheel well. Another would be to spread the cowl at the bottom to fit the wider frame rails, but if you want the body to sit flat on the frame without spreading the cowl you will need to pinch the rails. You can also narrow the entire frame from front to rear, but this creates a rather  wide overhang of the body through  the door area and therefore is not done very often. It really just depends on how you want to mount the body ..... this will determine whether or not you need to have the rails pinched.